Cast irons containing nodular graphite are well known in the art (Morrogh et al, J.I.S.I. 1970 (1948) 306) and many attempts have been made to predict the microstructure of cast iron, and in particular the nodularity of the graphite, by thermal analysis of test samples of iron melts. Backerud et al (The Metallurgy of Cast Iron, Georgi Publishing Co., Switzerland (1975) 625-637) studied Mg-treated nodular graphite iron by thermal analysis and found that the cooling curve for a vermicular/compacted graphite (CG) iron during the eutectic phase transformation falls between that of flake and nodular irons but that the eutectic recalescence for vermicular iron may exceed that of a flake or nodular iron.
Stefanescu et al, (Trans. AFS 90 (1982) 333-348) however, reported that the cooling curve of a vermicular/CG iron does not really lie between those of flake and nodular iron but rather crosses these curves several times. In these later studies, however, a rare earth or 0.5% cerium containing Mg Fe Si alloy was used as a nodulizer.
Loper et al (Metallurgy of Cast Iron, Proceedings of St. Saphorin Conference, Georgi Publishing Co. 1975, p. 640-657) studied the effect of NiMg treatment with and without postinoculation and reported that no eutectic recalescence occurred for a hypoeutectic nodular iron treated with NiMg without postinoculation, while a small eutectic recalescence was observed for hypereutectic nodular iron with the same treatment. After postinoculation, the overall eutectic temperature was raised but the eutectic temperature was decreased in a continuous manner for hypereutectic nodular iron. Monroe and Bates (Trans AFS, 90 (1982) 307-311) showed that solidification of Mg-treated irons at a eutectic temperature of about 1150.degree. C. indicates inadequate Mg treatment. Eutectic solidification at about 1132.degree. C. can be considered to reflect spheroidal graphite (SG) growth, while lower temperatures are indicative of massive carbide formation.
While it is known that the primary role of cerium in cast iron is to desulphurize the melt and then inhibit carbide decomposition, it is believed to have a secondary role in neutralizing "subversive" elements, such as bismuth, lead, and antimony present in the melt. Subversive elements all have an effect on graphite morphology but their effect on the cooling curves for nodular cast iron are unknown.
It will be appreciated, therefore, that both magnesium and cerium and/or trace elements have an effect on graphite morphology but the prior art teachings make it difficult to predict those effects accurately. It would be extremely useful to the founder to be able to predict the graphite morphology of a given melt of nodular iron before the melt is actually cast so that adjustments to composition etc. can be made so as to improve the quality of the final product.
There is, therefore, a need for a method and apparatus for predicting graphite morphology of a nodular iron before casting thereof.